We are wondering if there would be a new enum case for UIUserInterfaceIdiom for iPhone Duo or we should use the case .phone
iPhone Duo is still UIUserInterfaceIdiomPhone.
However, as mentioned in these videos:
"Modernize your UIKit app" https://developer.apple.com/videos/play/wwdc2026/278/
"Prepare your app for iPhone Duo" https://developer.apple.com/videos/play/tech-talks/111461/
You should audit all the locations in your app using UIUserInterfaceIdiom to gate layout or even functionality.
For example, multiple scene instances are supported on iPhone Duo, just like on iPad:
https://developer.apple.com/videos/play/tech-talks/111464/?time=220
So it is best to audit all uses of idiom in your app. The App Resizability Skill mentioned here can help identify and migrate those in your code base:
https://developer.apple.com/videos/play/tech-talks/111461/?time=566
